In the optical communications space, receivers based on coherent detection techniques have suffered disadvantages that have, to date, prevented successful deployment in “real-world” installed communications networks. One such limitation is that both the transmitted carrier signal and the local oscillator signal are generated by respective Tx and LO lasers, which, in the case of “real world” network systems, will be compact fiber or semi-conductor lasers which are subject to manufacturing and environmental variations. FIG. 1 schematically illustrates principal components and operations of a conventional semi-conductor laser of the type commonly used in communications networks.
As may be seen in FIG. 1, a conventional semi-conductor laser 2 comprises a laser diode 4 driven by a drive signal 6 supplied by a driver circuit 8. The laser diode 4 emits coherent light which is coupled into an optical waveguide 10. The frequency (or, equivalently, wavelength) of the emitted light is typically a non-linear function of the drive signal current and laser diode temperature. Other factors may also affect the frequency, depending of the design of the laser diode 4. In general, the driver circuit 8 receives a frequency setting signal 12 indicative of a desired laser frequency, and generates the drive signal current that is required to obtain this frequency. For ease of understanding the concepts, it is useful to consider a feedback loop 14 which taps the optical waveguide 10 and samples a small portion (e.g. −20 dB) of the laser diode output, enables the driver circuit 8 to dynamically adjust the laser drive signal 6 to minimize the error between a desired and actual frequencies. In the illustrated arrangement, the feedback loop 14 includes an optical frequency reference, such as an optical etalon, which is used to measure the frequency of the tapped laser diode output. With this type of arrangement, the feedback-loop 14 and driver circuit 8 cooperate to form an optical frequency locked loop (FLL) for controlling the laser diode 4. Real implementations of these reference concepts can use free space optics or back facet light to achieve efficient packages. The frequency setting 12 can take any of a number of forms. In some cases, the frequency setting 12 is a multi-bit digital word generated by an external controller unit (not shown in FIG. 1). An equivalent analog signal may also be used. Alternatively, the output frequency of the laser 2 can be controlled by varying parameters such as the temperature or injection current of the laser diode 4. Multistage sources can have several parameters such as phase shifts, couplings, currents, gains, losses, shifts, beats, injections or reflections to achieve frequency control. In either of these cases, the frequency setting signal 12 may be replaced by one or more suitable analog or digital control signals, which are indicative of a desired target parameter value.
A limitation of semiconductor lasers typically used in optical communications system is that the laser diode 4 control loop (i.e. the driver 8 and feed-back loop 14) is typically designed such that the average output frequency (over a period of 100 s of milliseconds or more) is moderately stable at a value which is nominally fixed by the frequency setting 12. A reference measurement device is not completely accurate nor stable, and deviates with perturbations such as varying temperature or mechanical deformation or aging.
Moreover, the driver 8 and feed-back loop 14 response is normally too slow to prevent short period frequency excursions due to laser linewidth, phase noise, and especially mechanical, optical or electrical transients. As a result, frequency variations of as much to ±400 MHz, at rates on the order to 50 KHz are commonly encountered. In a coherent optical receiver system, this problem is typically addressed by implementing an optical frequency locked loop (FLL) or Phase locked loop (PLL) to actively control the receiver's LO to match the received carrier signal. FLL and PLL circuits for this purpose are described in: “High Capacity Coherent Lightwave Systems”, Linke et al, Journal of Lightwave Technology, Vol. 6, No. 11, November 1988; “Heterodyne Phase Locked Loop by Confocal Fabry-Perot Cavity Coupled AlGaAs lasers”, Shin et al, IEEE Photonics Technology Letters, Vol. 2, No. 4, April 1990; and “Carrier Synchronization for 3- and 4-bit-per-Symbol Optical Transmission”, Ip et al, Journal of Lightwave Technology, Vol. 23, No. 12, December 2005. These systems operate to drive the receiver's LO to precisely track excursions of the received optical carrier. A limitation of this approach is that for optical communications systems with multi-gigabit line rates, an optical PLL/FLL loop bandwidth on the order of hundreds of MHz is needed to effectively compensate the laser phase noise. This is difficult to achieve at acceptable cost.
Heterodyne methods control the local laser to produce a target intermediate frequency carrier in the optical mixing product, and then use microwave electrical methods to beat that intermediate signal down to base-band. Heterodyne methods suffer from the requirement that the intermediate frequency be far from zero, indeed greater than the signal bandwidth, at least doubling the bandwidth required of the electrical receiver.
A newer approach is to use a complex electrical carrier recovery circuit for detecting and compensating a small frequency mismatch between the LO and received carrier. A complex carrier recovery circuit designed for this purpose is described in “Phase Noise-Tolerant Synchronous QPSK/BPSK Baseband-Type Intradyne Receiver Concept With Feedforward Carrier Recovery”, R Noé, Journal of Lightwave Technology, Vol. 23, No. 2, February 2005. A limitation of electrical carrier compensation in this manner is that it can only feasibly compensate some aspects of moderate frequency errors. As a result, a large frequency transient can cause severe performance degradations, for example due to limited analog amplifier bandwidth, and clock recovery issues.
A further limitation of prior art systems, is that they are highly vulnerable to optical impairments of the received carrier signal. In particular, in “real world” optical communications systems, optical dispersion on the order of 30,000 ps/nm and polarization rotation transients at rates of 105 Hz are commonly encountered. In the presence of such severe optical distortions, known LO synchronization and/or carrier recovery techniques tend to fail due, at least in part, to: failure of clock recovery and insufficient loop bandwidth of the Laser PLL/FLL and/or electrical carrier recovery circuits.
Accordingly, methods and techniques that enable Local Oscillator (LO) laser control in a coherent receiver unit of an optical communications network remain highly desirable.